Authors: Gregory Szypko (Rice University), Stephen Bradshaw (Rice University)

In spectral line measurements of the solar corona, the Doppler broadening often cannot be fully explained by thermal broadening at the presumed ion temperature. In this work, we investigate nanoflare heating as a possible cause of this excess (or “non-thermal”) line broadening. We focus on what we call the non-LTE “ratcheting” mechanism: nanoflare-driven non-LTE collisional heating cycles which successively drive ion temperatures above the electron temperature between nanoflares. In 2.5D MHD simulations using our code SPRUCE, we drive a closed-field coronal system with periodic nanoflare-scale heating events. We then post-process the output to simulate the resulting multi-species, non-LTE, non-equilibrium ionization evolution while also accounting for plasma time-history. With this approach, we synthesize excess line broadening values for several heavy ion species relative to the electron temperature. We synthesize these signatures at the nanoflare core and at several distances away from the nanoflare core. We find that non-LTE ratcheting is likely insufficient to explain the excess broadening values observed in the corona. However, we determine that this excess broadening is not sensitive to instrument line-of-sight alignment with the nanoflare site itself, which has important consequences for the interpretation of broadening measurements.